Append Not Working - Copy and Paste between files not reliable
I discovered what the problem was - it's very simple and is just a result of a slightly confusing UI.
On choosing the file to append, the blue append entry field remains blue while it loads up the file...but to me it looks like nothing is happening. That's because it takes a while to load up the file - sometimes 15 to 20 seconds. I am hitting the append sign because I am not being given any feedback that Blender is working, and it is trying to append when I have not made a selection.
It was bound to be something simple.